"I am deeply shocked and saddened by the outrageous terrorist attack in Boston yesterday. This senseless and cowardly act of violence has struck a city that has long stood as the symbol of openness, learning, innovation and enterprise," he wrote to Obama.
He said that the people of India joined him in condemning the attack in the strongest terms.
"We stand in solidarity and sympathy with the bereaved families, the injured and the people of the United States," Singh wrote.
At the same time, Singh sai, "it redoubles our resolve to remain unrelenting in our efforts to defeat terrorism and to defend and uphold the values that define our nations."
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Singh also offered full support to the US in the investigations into the attack.
"Mr. President, in keeping with the excellent cooperation between India and the United States to combat terrorism, we offer you our full support for the investigations into the attack," he wrote.
At least three persons were killed and more than 140 injured in two blasts near the finish line of the annual Boston marathon yesterday.